Generator Breaker and ATS withstand and close on rating

[PE (always learning)]

Hey Everyone,

I have a 100A generator breaker protecting an 80 amp Kohler Transfer switch and the 50 KW generator does not provide enough fault current to trip the breaker in the instantaneous region. I am wanting the breaker to trip in the instantaneous region so that I can apply the "any breaker" withstand and close on rating for the transfer switch, but because the generator doesn't produce enough fault current I don't think I will be able to apply this rating. For those of you that do power systems studies, what do you all normally recommend in a scenario like this? Also, attached is a picture of the TCC

 

[electrofelon]

I am wanting the breaker to trip in the instantaneous region so that I can apply the "any breaker" withstand and close on rating for the transfer switch

I am not clear on what you are trying to do. I understand that the transfer switch will have a SCCR and possibly different SCCR ratings depending on what type and size OCPD you have ahead of it, but where are you getting this bit about if instantaneous=any breaker? Also I would think the normal/utility side is where you will have much higher AFC so that is where you run into issues with SCCR first. I think some more information would help.

 

[PE (always learning)]

I'm referring to Kohler's "any breaker" rating for their transfer switches, sometimes it's also called a short time rating if I'm not mistaken. I have attached their withstand current ratings table to this email for your reference. The "any breaker" rating can be applied if you do not have a breaker that's listed on their "specific breaker" table. If you look at the table attached, I clouded in red what I'm referring to. You can use the "any breaker" rating if it clears the fault within the allowed time duration shown, but the generator being used is a 50 KW generator and doesn't allow fast clearing in the instantaneous region because the fault current is so low. Which leaves me with probably having to tell them to choose one of the "specific breakers".

Also, the breaker on the normal side of the transfer switch is already one of the breakers shown on their "specific breaker" table so I am covered for the withstand and close on rating on the normal side.

 

[d0nut]

Your 100A circuit breaker will not protect an 80A transfer switch.

 

[electrofelon]

Your 100A circuit breaker will not protect an 80A transfer switch.

I don't believe there is any general requirement to protect a switch at its rating.

 

[electrofelon]

I'm referring to Kohler's "any breaker" rating for their transfer switches, sometimes it's also called a short time rating if I'm not mistaken. I have attached their withstand current ratings table to this email for your reference. The "any breaker" rating can be applied if you do not have a breaker that's listed on their "specific breaker" table. If you look at the table attached, I clouded in red what I'm referring to. You can use the "any breaker" rating if it clears the fault within the allowed time duration shown, but the generator being used is a 50 KW generator and doesn't allow fast clearing in the instantaneous region because the fault current is so low. Which leaves me with probably having to tell them to choose one of the "specific breakers".

Also, the breaker on the normal side of the transfer switch is already one of the breakers shown on their "specific breaker" table so I am covered for the withstand and close on rating on the normal side.

Ok interesting. I don't recall ever seeing a trip time requirement for a SCCR rating. What is the AFC for the generator at the terminals of the xfer switch?

 

[ron]

Typically a 100A breaker cannot protect an 80A ATS.

Also, a generator rated to put out ~80A (that is my guess based on you having an 80A ATS), will not have a a subtransient reactance low enough to put out more than 10kA worth of fault current. Since your breaker has an instantaneous trip element, you comply, no need to ensure there is enough fault current to activate it, because it can't put out that much (circular logic), so you are ok.